Abstract

Leucitites from the Italian volcanic province show a marked enrichment of the large ion lithophile elements (LILE) over the high field strength elements (HFSE), indicative of a subduction-related origin. Despite Sr- and Nd-isotope ratios similar to those in lamproites, trace-element variations do not suggest the latter as a source for the K-enrichment. Instead, potassium and the other LILE and rare-earth elements (REE) are derived from subducted sediments in a multicomponent mixing process that involves both dehydration and partial melting of the subducted material. The slightly elevated δ18O and high 87Sr/86Sr values of the primitive Italian lavas are shown to be consistent with a three-component mixing model, which can also explain other unusual geochemical features of the Italian lavas. Stabilization of phlogopite in hybridized mantle above the subduction zone is regarded as an essential aspect of the petrogenesis of the orogenic leucitites.

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